<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>画三角形</title>
    <style>
        .trangle{
            width: 0;
            height:0;
            border: 40px solid red;
            border-top-color: black;
            border-bottom: none;
            border-left-color: transparent;
            border-right-color: transparent;
        }

    </style>
</head>
<body>
    <div class="trangle"></div>
</body>
</html>